Elasticsearch allows you to configure a scoring algorithm or similarity per
field. The similarity setting provides a simple way of choosing a similarity
algorithm other than the default TF/IDF, such as BM25.

Similarities are mostly useful for text fields, but can also apply
to other field types.

Custom similarities can be configured by tuning the parameters of the built-in
similarities. For more details about this expert options, see the
similarity module.

The only similarities which can be used out of the box, without any further
configuration are: